vout: allow the display modules to set a video context with the request chroma

Open Steve Lhomme requested to merge robUx4/vlc:display_vctx into master

If a display modules wants to request a GPU chroma when the source is a CPU chroma, it should be able to do so. In that case the core should use that video context to provide to filters (for example to handle the CPU to GPU conversion).

Ultimately display modules can just request GPU chromas and do the texture upload in a converter outside of the display module.

Followup to the discussion on !3436 (comment 409707)

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